Abstract

Seasonally dry tropical forests are dominated by deciduous and evergreen tree species with a wide range of leaf phenology. We hypothesized that Piscidia piscipula is able to extend leaf senescence until later in the dry season due to deeper and more reliable water sources than Gymnopodium floribundum, which loses leaves earlier in the dry season. Physiological performance was assessed as timing of leaf production and loss, growth, leaf water potential, depth of water uptake determined by stable isotopes, and leaf stable isotopic composition of carbon (δ13C) and oxygen (δ18O). P. piscipula took water primarily from shallow sources, whereas G. floribundum took water from shallow and deep sources. The greatest variation in water sources occurred during the onset of the dry season, when G. floribundum was shedding old leaves and growing new leaves, but P. piscipula maintained its leaves from the previous wet season. P. piscipula showed greater relative growth rate, greater leaf expansion rates, and more negative predawn and midday water potentials than G. floribundum. P. piscipula also exhibited greater leaf organic δ13C and lower δ18O values, indicating that the decrease in photosynthetic carbon isotope discrimination was associated with greater stomatal conductance and greater photosynthesis. Our results indicate that the contrasting early and late dry season leaf loss phenology of these two species is not simply determined by rooting depth, but rather a more complicated suite of characteristics based on opportunistic use of dynamic water sources, maximizing carbon gain, and maintenance of water potential during the dry season.

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Acknowledgments

We thank to CONACyT-UCMEXUS for the scholarship granted to the first author for doctoral studies at the University of California Riverside. This work was supported by UCMEXUS through the projects: “Importance of limestone bedrock and subsurface pockets of soil as potential sources of water for dry deciduous tree species in northern Yucatán” and “Water contribution by limestone bedrock and subsurface SP and its effect on the water use efficiency of two tree species in Yucatan, Mexico”; grants from the National Science Foundation (DEB 0615427, DEB 0770 6813, EF 0410408, and CRR 0120778), and the UCR Center for Conservation Biology. We would like to thank the owners of the quarry, especially Héctor Buenfil Cervera; the workers of the quarry, especially Roger Armando Fernández Paredes, and Augusto Cervera Buenfil for helping with the work logistics; Alfonso Castillo, María José Romero, Raúl Hernandez and Ariel Esparza for helping with the field and laboratory work.